The invention concerns a process for the production of linear segmented polyurethanes through simultaneous reaction of macrodioles, aromatic diisocyanates and low-molecular monomeric diols as chain-lengthener in an inert solvent according to the so-called one-pot process in the presence of catalysts, polyurethane produced according to this process, as well as uses of such polyurethanes, particularly for the production of membranes.
Polyurethanes belong to a class of polymers which distinguishes by a very great variety. Appropriate choice of starting materials provides practically unlimited possibilities for variation in production. Numerous aliphatic, cycloaliphatic and aromatic polyisocyanates can be used alone or in mixture for construction of the polyurethane. In addition to low-molecular monomeric glycols such as ethyleneglycol, butyleneglycol, also higher molecular weight substances displaying hydroxy end-groups, such as polyester, polyglycol and the like can be employed or used together. By varying the ratio of low-molecular glycol to higher-molecular glycol polyurethane of the most different characteristics can be produced. Polyurethanes, in which building blocks are present in the polymer chain that are composed of the higher-molecular polyester or the higher-molecular glycol and which moreover are connected with segments that arise through the reaction of diisocyanate with the lower-molecular glycol, such as, for example, ethyleneglycol, are also called block polymers or segmented polyurethane.
The multitude of polyurethanes can be increased further through the use of compounds with more than two functions, such as e.g. trifunctional alcohols.
Beyond that, side reactions frequently occur to more or less great extent with the synthesis of the polyurethane, such as e.g. the formation of allophanate, whereby intended or unintended the characteristics of the polyurethane are altered. The number of possibilities in polyurethane chemistry is thereby practically inestimable.
It is also of influence in the production of polyurethane whether it is produced in a mass, that is without use of a solvent, or whether the reaction is performed in a solvent.
When working in a solvent it is possible to proceed in many different ways, including initially preparing a so-called pre-polymer by reaction of a macrodiol with a diisocyanate, and then reacting this with a chain-lengthener. It is often desired to utilize the so-called one-pot process, with which all starting materials, i.e. the polyisocyanate, the macrodiol and the low-molecular glycol are simultaneously reacted. Further possibilities for variation exist when, using the one-pot process, the ratio of NCO to OH groups is variably chosen during the main reaction.
With non-catalyzed one-pot reactions it is frequently necessary to work with an excess of diisocyanate in order to achieve the desired molecular weight. This, however, promotes undesirable side reactions, e.g. allophanate and biuret formation. As a result, the reproducibility of the lengthening is impaired, while separations can also occur.
Influencing factors further to the numerous above mentioned possibilities for variation exist, aside from choice of starting materials. These depend on the use of the most different catalysts. Numerous catalysts are known which have been used for the production of polyurethane. With these not only can the speed of the polyurethane formation be increased, but also, intentionally or unintentionally, the characteristics of the produced products can be altered. Thus, through the use of the most different catalysts, the molecular weight of the product is frequently altered during production of polyurethane. Likewise, molecular weight distribution can be influenced. In addition to catalyzing, the catalysts used not only lead to particular polyurethane formation, but frequently also more or less the most different side reactions such as allophanate formation, trimerisation, and the like, which result in branched, networked and thereby generally heavy or even insoluble products. For mass production, fluctuations in the characteristics of the polyurethane, frequently within relatively broad limits, can come into play. For many purposes of use a product is desired with characteristics precisely adjustable and as uniform as possible. This applies in particular to the field of membranes, the production of which from a prepared polymer has already promoted a relatively complicated technology. Thus, the characteristics of membranes are influenced in great measure through deviations in the thickness of the foils, through coagulation conditions, or through their structure, so that one is not confronted with yet a further uncertainty, namely the structure of the polymerisate. There is, moreover, great interest in providing polymers with constant chemical structure and constant characteristics.
It has already been attempted to produce linear segmented polyurethanes with constant viscosity by adding to the reaction mixture, either from the outset or at a determined stage of the reaction, so-called chain-terminating agents and stabilizers. In this manner, however, it is not possible to obtain reproducible polyurethanes with the same, and as narrow as possible molecular weight distributions. Even by varying the reaction temperature such is achieved only to an unsatisfactory measure. At lower temperatures the reactions take far too long in order to permit an industrially useful production of polyurethane solutions. At higher temperatures both side reactions and construction reactions occur to an undesirable extent. This problem has not previously been solved even through the use of an entire series of catalysts. Numerous catalysts, in particular the very frequently used tertiary amine compounds, no longer display catalytic effectiveness in polar solvents at higher temperatures. Many catalysts, such as e.g. organic tin compounds, are toxic and therefore little suitable for polyurethane membranes used for medically related purposes.
Numerous catalysts are mentioned in the article by L. Thiele "Isocyanatreaktion und Catalyse in der Polyurethanchemie", Act. Polymerica 30 (1979), Vol. 6, pp. 232-242, which can be used for the production of polyurethanes. The catalysts mentioned there, however, do not provide the advantages according to the present invention.
U.S. Pat. No. 3,769,245 describes the production of polyurethane foams in the melt. Isocyanate groups are supposed to be reacted with carboxyl groups, and as catalyst preferably magnesium acetate should be used. Magnesium acetate is however completely unsuitable as catalyst for the process according to the present invention.
Acids mentioned, which form with the most different metals salts that are supposed to be suitable for the theredescribed process, are predominantly aliphatic acids, which likewise provide no catalysts useful for the process according to the present invention. Indication that the peripherally mentioned benzoates and naphthates, particularly calcium- or magnesium benzoate or -naphthate, should make possible the production of polyurethanes or polyurethane solutions with the desired characteristics, is not to be found in this reference.
Although already numerous processes for the production of polyurethanes are known, there still exists a need for an improved process for the production of polyurethanes which distinguish particularly by a narrow molecular weight distribution.